In my degree whenever we were to analyze a sample of English they were always careful to specify whether it was Australian English, American English, Singapore English (Singlish) or Malaysian English (Manglish). Any sample marked as simply 'English' was considered English as it was spoken and pronounced in England.
There are enough differenes in the languages for a push to consider them totally separate languages but my personal opinion is that rather than calling the differences 'language differences' (or calling them totally separate languages with the same roots), I think it's more appropriate to call them regional differences.
American English's regional differences are the Boston or New York ways of talking (like George Costanza or Mayor Quimby from The Simpsons), or saying 'y'all' in the South, and Australian English's differences are things like 'pint' in the South and 'schooner' in the north (both meaning a large glass of beer) and numerous phonetic differences (producing vowels slightly differently).
Every English-speaking country has its differences in lexical items and pronounciation (New Zealand's an obvious one with their 'fush and chups') but they're still all part of New Zealand English or Australian English or American English. The differences in how Americans speak and how Australians speak are similar to these regional differences, since it's all English and the differences are minimal so it's not all mutually unintelligible. That's why I think that it's all the English language and that American English and Australian English and New Zealand English are regional variations and not actual languages.
But it's a bit of a sticky situation because there is still a lot of debate about where to draw the line between dialect and language and regional variation of a same language and whether the same root language should be a factor and whether people can understand each other should be considered... all sorts of fun stuff.
But I do believe that Singlish and those sorts of languages should be considered their own languages, not under the English umbrella and not under the Mandarin, Japanese, Malay etc. umbrellas either. Singlish has its own fuctional grammar and its own language rules and although it's got bits from English and bits from Singaporean languages I think it's more than just a creole.
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